MRI devices utilize large permanent magnets to generate the very substantial magnetic fields to which the patient is exposed. Successful imaging requires that the magnetic field have predictable characteristics and be as uniformed as possible. A typical MRI magnet utilizes a pair of planar magnets of opposite polarity which are maintained in an opposed, spaced-apart relationship by a supporting frame, so that the patient may be placed in the gap between the two magnets. The frame is typically U-shaped in cross-section, with the magnets being mounted on the inside of the opposite legs of the U. The frame is made of a highly permeable magnetic material, such as steel, so as to provide a return path for the magnetic flux between the two magnets.
During the construction of the magnet structure, great care is taken to mount the magnets so as to achieve the desired uniform field. However, the strong field between the magnets produces a very large force, which tend to pull the two arms of the U-frame together. If this force were to so distort the frame, the relative orientation of the two magnets would be changed and the uniformity of the magnetic field would be adversely affected. In order to avoid such distortion of the frame, it is constructed so as to be particularly heavy and rigid. Unfortunately, this often makes the magnet structure bulky and difficult to handle, as well as expensive.
Broadly, it is an object of the present invention to provide a magnet structure useful for MRI devices which avoids the shortcomings of known structures of this type. It is specifically contemplated that a substantially smaller and lighter structure be provided so as to be readily transported and moved into different locations and positions for use.
It is also an object of the present invention to provide a permanent magnet structure useful in MRI devices which is reliable and convenient in use, yet relatively inexpensive in construction.
In accordance with the present invention, a permanent magnet structure includes a generally U-shaped frame made of a magnetic material and a pair of opposite polarity magnet members mounted on the opposed surfaces of the legs of the U. The connecting element of the U, which connects the legs, is disposed at a sufficient distance from the magnets so that the resulting magnetic field passes substantially entirely between the magnets and through the arms and so that no substantial eddy currents are induced in the connecting element by the imagining system gradient coils during imagining. Between the connecting element and the magnets, there is disposed a non-conductive, non-magnetic separating member which contacts the two arms and tends to keep them apart. The separating member is so positioned relative to the connecting member and the magnets that affects the force between the two magnets and the force produced by the magnetic field passing through the arms and connecting element are counterbalanced. This results in a structure which is mechanically stable.